Orthopedic device for repair or replacement of bone

ABSTRACT

Prosthetic devices for repair of replacement of bone structure in a living body, and methods of orthopedic repair which employ such devices. The orthopedic devices comprise a substrate and a pyrolytic carbon coating on the substrate, which pyrolytic carbon coating is compatible with living tissue and which has a density of at least about 1.5 grams per cubic centimeter. Examples of suitable substrates are those which have a modulus of elasticity approximating that of natural bone such as polycrystalline carbon, and fiber aggregates such as carbon fiber aggregates and refractory wire metal screens. The pyrolytic carbon coating of the orthopedic devices may be polished to provide an effective wear surface, while the surface roughness of the as-deposited coating may be employed to achieve a bond with natural bone tissue. The pyrolytic carbon coating is preferably isotropic and may be doped with a suitable carbide-forming element, such as silicon, to provide additional structural strength and wear resistance.

United States Patent Bokros et al.

[54] ORTHOPEDIC DEVICE FOR REPAIR OR REPLACEMENT OF BONE [72] Inventors:Jack C. Boltros, San Diego; Willard R. Ellis, Leucadia, both of Calif.

[73] Assignee: Gull Oil Corporation, San Diego,

Calif.

[22] Filed: Aug. 26, 1970 [211 App]. No.: 67,148

Related U.S. Application Data [63] Continuation-in-part of Ser. No.649,811, June 29,

1967, Pat. No. 3,526,005.

[52] us. 01. ..3/1, 128/92 c, 128/92 CA, 32/10 A, 117/46 ca [51] 1m. c1..A6lt1/24 [58] Field of Search...3/l; 128/1, 92 C, 92 CA, 92 R, 128/92BC; 32/10 R, 10 A; 117/46 CG, 46 CB, 46 CC; 23/209.1, 209.2, 209.4

[5 6] References Cited UNITED STATES PATENTS 3,526,906 9/1970 De Laszlo..3/1 3,298,921 1/1967 Bokros et a1 ..1 17/46 CG I 3,178,728 4/1965Christensen ..3/l 3,314,420 4/1967 Smith et a1 ..128l92 C 3,064,645 1H1962 Ficat et al. ..128/92 CA FOREIGN PATENTS OR APPLICATIONS 49,63911/1889 Germany ..32/1OA OTHER PUBLICATIONS Ethicon Tantalum Gauzeadvertisement page 4 by Ethicon Suture Laboratories, lnc., NewBrunswick,

N;J.-, Surgery, Gynecology & Obstetrics, Sept. 1951.

Vitallium Surgical Appliances Catalog, by Howmet Corp. Austenal MedicalDivision, N.Y., N.Y., 1964, Maclntosh Tibial Plateaus (No. 6958-5) onpage 61 relied upon.

Primary Examiner-Richard A. Gaudet Assistant Examiner-Ronald L. FrinksAttorney-Anderson, Luedeka, Fitch,Even and Tabin f [57] ABSTRACT I 1Prosthetic devices for repair of replacement of bone structure in aliving body, and methods of orthopedic repair which employ such devices.The orthopedic devices comprise a substrate and a pyrolytic carboncoating on the substrate, which pyrolytic carbon coating is compatiblewith living tissue and which has a density of at least about 1.5 gramsper cubic centime- 1 ter. Examples of suitable substrates are thosewhich have a modulus of elasticity approximating that of natural bonesuch as polycrystalline carbon, and fiber aggregates such as carbonfiber aggregates and refrac- Jtory wire metal screens. The pyrolyticcarbon coating of the orthopedic devices may be polished to provide aneffective wear surface, while the surface roughness of the as-depositedcoating may be employed to achieve a bond with natural bone tissue. Thepyrolytic carbon coating is preferably isotropic and may be do ed with asuitable carbide-formin element such as silicon, to provide additionalstructu ral strength and wear resistance.

14 Claims, 7 Drawing Figures ORTHOPEDIC DEVICE FOR REPAIR OR IREPLACEMENT OF BONE This application is a continuation-in-part ofcopending application Ser. No. 649,8l l, filed June 29, 1967, now US.Pat. No. 3,526,005.

The present invention relates to prosthetic devices designed for use inorthopedics. More particularly, the present invention relates torepairing orthopedic defects by using prosthetic devices designed forsuch purposes.

The use of prosthetic devices for repair or replacement of bonestructure in a living body is well known. Conventional prostheticdevices have been constructed from metals, ceramic, and plastics,depending upon their intended application. Their development has beenspurred by the intense human need peculiar to an adverse medicalsituation; such devices, however, are only partially or temporarilyadequate and have various serious defects which limit their use tosituations of such seriousness that their substantial disadvantages areoutweighed.

One requirement for a satisfactory prosthetic device which is to bepermanently implanted is that it be physiologically inert for indefiniteperiods of time, and one difficulty with conventional devices is thatthe materials from which they are constructed may be physiologicallyrejected, may cause inflammation of bodily tissue or may be degraded bybodily processes. For example, metals may corrode to cause structuralweakness and pathology from the corrosion products. Plastics, even thoseextremely chemically inert, such as polytetrafluoroethylene may bedegraded in the body over long periods of time, or cause inflammation orabnormal cellgrowth.

Furthermore, the stresses developed within the skeletal framework of aliving body, which is naturally designed to serve a load-bearingfunction, provide considerable difficulties of a mechanical andstructural nature. In addition, the functional interrelationship betweenthe skeleton and the rest of the body is of such a biologicallyintricate nature that even more complex difficulties confront thedevelopment of suitable prosthesis. For example, joint prostheses areconventionally constructed from metals or plastics. Not only do plasticstend to be degraded in the body, but they also exhibit poor wearcharacteristics under the severe mechanical performance requirements fora joint prosthesis. Metal-metal joint prostheses tend to gall and wear,and the metallic dust created thereby may require re-operation. Metallicprostheses which replace only part of a functional joint may be so rigidthat they cause damage to the remaining, natural portions of the oint.

These problems are particularly acute for skeletal regions which arenormally subjected to considerable stress. For example, conventional hipjoint ball prosthesis may appear to operate satisfactorily for up toseveral years; however, such prostheses generally tend to eventuallycause thickening, inflammation, and pain in the synovial lining andcapsule of the joint.

Replacement of one articulating portion of a joint with a rigid,metallic prosthesis may have a deleterious effect upon the tough,elastic cartilage which covers the remaining, articulating end of thenatural portion of the joint. Contact and wearing, particularly undercon-' ditions of load-bearing or stress, of a hard metallic prosthesisagainst a portion of natural bone will usually result in damage to theliving bone tissue. Furthermore, under the conditions of stressassociated with skeletal functionality, conventional metallic, rigidprostheses exhibit considerable difficulty in remaining permanentlyimplanted in a supportive function in living bone. Such metallicprostheses have so high a modulus of elasticity that they do not flex inharmony with the bone in which they are implanted, but rather, becausethey are more rigid than the bone,'concentrate stress in portions of theremaining, lower modulus bone, particularly at the metal-bone interface.Thus, not only do conventional metallic prosthesis exhibit theundesirable characteristic of subjecting remaining portions of naturalbone to unnatural, concentrated stress, but they also have theadditional disability of being difficult to anchor in the natural bonebecause stress concentration at the interfaces between the bone and theprosthesis will cause the prosthesis to toggle or otherwise work loosefrom its fastening to the bone.

When a segment of bone is missing due to accident or surgery, there isat present no satisfactory method other than bone homograft to replacethe missing bone segment and join the remaining portion of the bone.Attempts have been made to use porous ceramics to act as a skeleton orframework into which new bone might grow, so that the missing segment ofbone would be replaced partially by natural processes. Such approachesemploying porous ceramic prostheses have been generally unsuccessful,not only because such materials are often not accepted by the body butalso because the very objective of fostering natural bone growth withinthe ceramic has not been successfully attained.

Metallic dental prostheses which abut or penetrate the jaw bones alsosuffer the disabilities and shortcomings associated with metalprostheses in general, and dental repair is still conventionallyaccomplished by extracorporeal support fromremainin g natural teeth orgums.

Prosthetic and orthopedic devices have been used for a number of years,and it is expected that usage of such devices will increase in thefuture as medical expertise continues to improve. The provision ofimproved prosthesis for repair or replacement of bone structure in aliving body would greatly facilitate the development of that medicalexpertise and alleviate considerable human suffering.

It is an object of the present invention to provide improved prostheticdevices. Another object is to provide prosthetic devices which arephysiologically compatible with body tissues although implanted in thebody for long periods of time. A further object of the invention is toprovide prosthetic devices which perform exceptionally well in thedemanding mechanical environment of the skeletal structure of a livingbody, and which are capable of establishing a firm bond thereto. Stillanother object of this invention is to provide prosthetic devices whichare eminently suited for extended functional performance as, forexample, joint prostheses, dental prostheses, whole or partial bonereplacement prostheses, and framework-like prosthesis into andthroughout which natural, living bone tissue may grow so that repair orreplacement of bone structure may be partially accomplished by naturalprocesses. An additional object is to provide improved prostheses whichhave a modulus of elasticity approximating that of natural, living bone,and which do not concentrate substantial stress at the prosthesis-boneinterface, but rather flex in harmony with the natural bone underapplied stress. A still further object of this invention is to provide amethod of repairing an orthopedic defect byusing prosthetic devicesthrough which the qualities of natural living bone may be closelyapproximated,

These and other objects of the invention are more particularly set forthin the following detailed description and in the accompanying drawingsof which FIG. 1 is a perspective view partially broken away,'of

a knee joint prosthesis;

FIG. 2 is a bottom view. of the knee joint prosthesis of FIG. 1;

FIGS. 3 and 4 are perspective views of dental prostheses adapted to beimplanted into a jaw bone of a living body;

FIG. 5 is a perspective view, partially broken away,

of a hip joint prosthesis, and

. FIG. 6 is a bone framework prosthesis. into and throughout whichliving bone tissue may grow so that replacement of bone structure in aliving body may occur partially through natural bone growth,

FIG. 7 is a side elevation, partially broken away, of the bone frameworkprosthesis of FIG. 6 in position between two segments of natural bone.

It has been found that prosthetic devices having improvedcharacteristics can be made by coating suitable substrates'of thedesired shape and size with pyrolytic carbon. Pyrolytic carbon iscapable not onlyof significantly increasing the strength ofthe substrateupon which it is coated, but it is also able to resist wear anddeterioration even when implanted within a living body for long periodsof time. While reference is hereinafter generally made to the use ofprosthetic devices for repair or replacement of bone structure in aliving human body, it should also be recognized that the improvedprosthetic devices and orthopedic processes may-also have veterinaryapplications in other living animals which have an internal skeletalstructure. For example, it may be desirable to use prosthetic deviceshaving the indicated. pyrolytic carbon coatings for use in orthopedicrepair or replacement of bone in horses,

' dogs, or other domestic animals.

In general, the pyrolytic carbon coating is applied to a suitablesubstrate material so that it covers at least a major portion of thesurface thereof. The thickness of the pyrolytic carbon coating should besufficient to provide the necessary strength for its intended use, andoften it is desirable to employ the coating to impart additionalstrength to the particular substrate being coated. The coating will beat least 25 microns thick and usually at least about 50 microns thick.If a fairly weak substrate is being employed, for instance, one made ofartificial graphite, it may be desirable to provide a thicker coating ofpyrolytic carbon to strengthen the composite prosthetic device.

Moreover, although an outer coating which is substantially entirelyfairly dense pyrolytic carbon has adequate structural strength, thecodeposition of silicon or some other-carbide-forming additive may beemployed to improve the strength and wear resistance of the carboncoating. As described in more detail hereinafter, silicon'in an amountup to about 20 weight percent can be dispersed as SiC throughout thepyrolytic carbon without detracting from the biologically compatibleproperties of the pyrolytic carbon.

For use on complex shapes and in order to obtain maximum structuralstrength, it is desirable that a pyrolytic carbon coating on thesubstrate be nearly isotropic. Anisotropic carbons tend to delaminatewhen complex shapes are cooled after depositing the pyrolytic coating athigh temperatures. Thus, for coating complex shapes (i.e., those havinga radius or radii of curvature less than V4 inch), the pyrolytic carbonshould have a BAF (Bacon Anisotrophy Factor) of not more than about 1.3.For non-complex shapes, higher values of BAF up to about 2.0 may beused, and for flat shapes, pyrolytic carbon having a BAF as high asabout 20 may be used. The BAF is an accepted measure of preferredorientation in the layer planes in the carbon crystalline structure. Thetechnique of measurement and a complete explanation of the scale ofmeasurement is set forth in an article by G.E. Bacon entitled A Methodfor Determining the Degree of Orientation of Graphite which appeared inthe Journal of Applied Chemistry, Vol 6, p. 477, (1956). For purposes ofexplanation, it is noted that 1.0 (the lowest point on the Bacon scale)signifies perfectly isotropic carbon, while higher values indicateincreasing degrees of anisotrophy.

The density of the pyrolytic carbon is considered to be an importantfeature in determining the additional strength which the pyrolyticcarbon coating will provide the substrate. The density is furtherimportant in assuring tissue compatibility, and sufficient wearresistance of the prosthetic device where appropriate. It is consideredthat the pyrolytic carbon should at least have a density of about l.5grams per cubic centimeter,

and preferably the pyrolytic carbon has a density between about 1.9grams/cm and. about 2.2 grams/cm.

A further-characteristic of the carbon which also affects its structuralstrength contribution is the crystallite height or apparent crystallitesize. The apparent crystallite size is herein termed L and can beobtained directly using an X-ray diffractometer. In this respect L,=o.s9 was cos 0 wherein:

A is the wavelength in Angstroms B is the half-height (002) line width,and

0 is the Bragg angle. Pyrolytic carbon coatings for use in prostheticdevices should have a crystallite size no greater than about 200 A. Ingeneral, the desirable characteristics for pyrolytic carbon for use inprosthetic devices are greater when the apparent crystallite size issmall, and preferably the apparent crystallite size is between about 20and about 50 A.

Since the substrate material .for the prosthetic device willpreferably'be completely encased in pyrolytic carbon, choice of thematerial from which to form the substrate is not of utmost importanceper se, However, the substrate material should have the requisitemechanical strength and structural properties for the particularprosthetic application for which it is going to be employed. However, ifthe prosthetic device in use will have portions of the substrate exposedto bodily tissues, for example, as might occur from machining theprosthetic device into final form after the basic shape has been coatedwith pyrolytic carbon, the substrate should be selected from materialswhich are relatively biologically inert, preferably artificial graphite.

It is very important that the substrate material be compatible withpyrolytic carbon, and more particularly that it be suitable for use inthe process conditions for coating with pyrolytic carbon. Although it isdesirable that the substrate material have sufficient structuralstrength to resist possible failure during its end use, materials whichdo not have sufficiently high structural strengths (by themselves) maybe employed by using the pyrolytic carbon deposited thereupon to supplyadditional structural strength for the prosthetic device.

Because pyrolytic carbon is, by definition, deposited by the pyrolysisof a carbon-containing substance, the substrate will be subjected to thefairly high temperatures necessary for pyrolysis. Generally,hydrocarbons are employed as the carbon-containing substance to bepyrolyzed, and temperatures of at least about l000C. are used. Someexamples of the deposition of pyrolytic carbon to produce coatedarticles having increased stability under high temperature and neutronradiation conditions are set forth in U.S. Pat. No. 3,298,92l. Processesillustrated and described in this U.S. patent employ methane as thesource of carbon and utilize temperatures generally in the range fromabout 1500 to 2300C. Although it is possible to deposit pyrolytic carbonhaving the desired properties with regard to the instant invention atsomewhat lower temperatures by using other hydrocarbons, for example,propane or butane, generally it is considered that the substratematerials should remain substantially unaffected by temperatures of atleast about 1000C., and preferably by even higher temperatures. Thepyrolytic carbons deposited either with or without silicon attemperatures below about l500C. are particularly suited for use inprosthetic devices because such pyrolytic carbons have exceptionaltissue and bone compatibility, wear resistance, mechanical strength, andin combination with a suitable substrate will provide a prosthesis witha modulus of elasticity which is close to that of bone.

Because the substrate is coated at relatively high temperatures and theprosthetic device will be employed at temperatures usually very close toambient, the coefficients of thermal expansion of the substrate and ofthe pyrolytic carbon deposited thereon should be relatively close toeach other if the pyrolytic carbon is to be deposited directly upon thesubstrate and a firm bond between them is to be established. While theabove-identified U.S. patent contains a description of the deposition ofan intermediate, low density, pyrolytic carbon layer, the employment ofwhich might provide greater leeway in matching the coefficients ofthermal expansion, it is preferable to deposit the pyrolytic carbondirectly upon the substrate and I thereby avoid such an additionalintermediate layer. Pyrolytic carbon having the desired characteristicscan be deposited having a thermal coefficient of expansion in the rangeof between about 3 and about 6 X l0'/C. measured at 20C. Accordingly,substrate materials are chosen which have the aforementioned stabilityat high temperatures and which have thermal coefficients of expansionwithin or slightly above this general range, for example up to about 8 Xl0'/C. Examples of suitable substrate materials include artificialgraphite, boron carbide, silicon carbide, tantalum, molybdenum,tungsten, and various ceramics, such as mullite.

Prosthetic devices which are intended to replace a significant amount ofbone tissue without partial replacement by means of natural bone growth,preferably employ artificial graphite as the substrate material becausethese materials have a modulus of elasticity of from about 2 to about 4X 10 psi., which is ordinarily in the range of thatof natural livingbone. For example, a particularly preferred form of graphite for use asa substrate material is polycrystalline graphite. An example of such agraphite is the polycrystalline graphite sold under the trade name POCOAXF Graphite, which has a density of about 1.9 grams per cubiccentimeter, an average crystallite size (L,,) of about 300 A, anisotropy of nearly 1.0 on the Bacon scale, and a modulus of elasticityof about 1.7 X 10 psi.

When it is desired to provide a framework into and through which newbone tissue may grow, for example, in order to firmly attach aprosthetic device to a bone, or in order to replace or repair bonestructure partially through replacement by new, natural bone tissue,.ashereinafter described, substrates such as suitable metallic or carbonscreens, wires or fibers may be employed as the substrate material. Forexample, a screen formed from an alloy of 50 percent molybdenum and 50percent rhenium may be used. Screens of tantalum, tungsten, molybdenumor alloys thereof, which are preferably coated with tungsten to preventembrittlement during the coating with pyrolytic carbon, may also beused. Fiber aggregates in addition to screens such as non-woven feltedstructures and filamentwound structures may also be used as substratematerials.

The pyrolytic carbon coating is applied to the substrate using asuitable apparatus for this purpose. Preferably, an apparatus isutilized which maintains a substrate invmotion while the coating processis carried out to assure that the coating is uniformly distributed onthe desired surfaces of .the substrate. A rotating drum coater or avibrating table coater may be employed. When the substrates to be coatedare small enough to be levitated in an upwardly flowing gas stream, afluidized bed coater is preferably used. When larger substrates areemployed, or where it is desired to vary the thickness or othercharacteristics of the pyrolytic carbon coating over different portionsof the substrate, different coating methods may be employed, such assupporting the substrate on a rotating or stationary mandrel within alarge fluidized bed.

As discussed in detail in the above-identified United States patent, thecharacteristics of the carbon which is deposited may be varied byvarying the conditions under which pyrolysis is carried out. Forexample, in a fluidized bed coating process wherein a mixture of ahydrocarbon gas, such as methane, and an inert gas, such as helium orargon, is used, variance in the volume percent of the hydrocarbon gas,the total flow rate of the fluidizing gas stream, and the temperature atwhich pyrolysis is carried out, all affect the characteristics of thepyrolytic carbon which is deposited. Control of a strong base isotropicpyrocarbon coating, having a BAF of 1.3 or less, and near the end of thecoating operation, the coating conditions can be gradually changed toobtain a highly oriented outer layer. Using this technique, suitablecoatings having outer surfaces which are highly anisotropic and, forexample, are about 25 microns thick, can be conveniently deposited.

Generally, when pyrolytic carbon is deposited directly upon the surfaceof the substrate material, the pyrolysis conditions are controlled sothat the pyrolytic carbon which is deposited has a coefficient ofexpansion matched to within plus or minus 25 percent of the coefficientof expansion of the substrate material, and

. preferably to within about plus or minus 20 percent.

Because pyrolytic carbon has greater strength when placed in compressionthan when placed in tension, the thermal coefficient of expansion of thepyrolytic carbon is most preferably about equal to or less than that ofthe substrate. Under these conditions, good adherence to the substrateis established and maintained during the life of the prosthetic devices,and upon cooling of the pyrolytic coating-substrate composite, thepyrolytic carbon coating is placed in compression under conditions ofits intended use at about ambient temperature.

As previously indicated, the coating may be substantially entirelypyrolytic carbon, or it may contain a carbide-forming additive, such assilicon, which has been found to enhance the overall mechanicalproperties of the coating. Silicon in an amount of up to about 20 weightpercent, based on the total weight of silicon plus pyrolytic carbon, maybe included without detracting from the desirable physiologicalproperties of the pyrolytic-carbon, and when silicon is used as anadditive, it is generally employed in an amount between about 10 andabout weight percent. Examples of other carbide-forming elements whichmight be used as additives in equivalent weight percents include boron,tungsten, tantalum, niobium, vanadium, molybdenum, aluminum, zirconium,titanium, and hafnium. Generally, such an element would not be used inan amount greater than 10 atom percent, based on the total atoms ofpyrolytic carbon plus the element.

The carbide-forming additive is co-deposited with the pyrolytic carbonby selecting a volatile compound of the element in question andsupplying this compound to the deposition region. Usually, the pyrolyticcarbon is deposited from a mixture of an inert gas and a hydrocarbon orthe like, and in such an instance, the inert gas is convenientlyemployed to carry the volatile compound to the deposition region. Forexample, in a fluidized bed coating process, all or a percentage of thefluidizing gas may be bubbled through a bath of methyltrichlorosilane orsome other suitable volatileliquid compound. Under the temperature atwhich the pyrolysis and co-deposition occurs, the particular elementemployed is converted to the carbide form and appears dispersed as acarbide throughout the resultant product. As previously indicated, thepresence of such a carbide-forming additive does not significantlychange the crystalline structure of the pyrolytic carbon deposited fromthat which would be deposited under the same conditions in the absenceof such an additive. Pyrolytic carbon having the physical propertiesmentioned hereinbefore, is considered to be particularly advantageousfor constituting the surface for a prosthetic device because it is inertto the metabolic processes, enzymes and other juices (physiologicalfluids) found within living bodies. in addition, such pyrolytic carbonis not injurious to natural bone growth. Particularly preferred ispyrolytic carbon which has a density between about 1.9 and about 2.2grams per cubic centimeter, wherein the superficial porosity of suchcarbon facilitates the growth of bone thereto, which is particularlyconducive to natural bone growth so that the creation of .a good bondbetween natural bone and the prosthetic device may be established. Inorder to enhance attachment of either bone or tissue, the surface may beoxidized. This can be done to l) enhance the superficial roughnessand/or to 2) provide oxygen-bearing polar groupson the surface which mayreact with tissue or bone molecules.

The pyrolytic carbon surface of the prosthetic device may be fabricatedwith different physical properties at v different surface locations, forexample a dense, polished wear surface may be employed at thearticulating end of a joint prosthesis while a surfacehaving a rough,more porous surface may be employed atthe surfaces at which theprosthesis is joined with natural bone to facilitate bone growththereto.

In addition to mechanical modification of the pyrolytic carbon surfacesuch as polishing, it may be desirable to utilize other physical orchemical modifications of the pyrolytic carbon surface. For example,chemisorbed gases, such as oxygen, may be removed to provide a morehydrophobic surface, or conversely, a surface having an adsorbed gas,such as oxygen, or which is chemically modified such as by formingsurface hydroxyl groups, may be employed to provide a more hydrophilicsurface. It is believed that pyrolytic carbon surfaces which are morehydrophilic, such as those having surface hydroxyl groups, are moreconducive to the establishment of a firm bond with natural bone tissue.

In addition, the prosthetic device should be sterile before implantationin a living body. The device may be sterilized and chemisorbed oxygenremoved by heating in a vacuum at an elevated temperature. For example,the pyrolytic carbon-coated prosthesis may be ultrasonically cleaned inbenzene, and then again in distilled water, and then out-gassed at 900C.for two hours to effect complete sterilization and to remove ab sorbedgases and provide a hydrophobic surface. The device may be sterilizedalso by heating in a suitable vacuum for about 6 hours at C. or by steamautoclaving. The prosthetic device can also be sterilized replacementfor another bone segment, or as a dental prosthesis, known surgical anddental procedures are employed. It is recognized that provision of theimproved prosthesis of this invention will likely result in developmentof improved medical techniques of orthopedic repair. The prostheticdevices may be secured in the proper location within the body bysuitable means and procedures, including those which are hereafterdescribed for specific embodiments of the present invention.

Illustrated in FIG. 1 of the drawings is a knee joint prosthesis 10 forreplacing damaged knee joints. The knee joint prosthesis 10 is formedfrom a substrate 12 upon which is deposited a pyrolytic carbon exteriorcoating 14. The prosthesis 10 has a pivotal wear surface 16 which isarcuate in one direction between a front surface 26 and a rear surface28 and which is designed for pivoting against another such arcuate wearsurface in the functioning of a knee joint. The wear surface 16 is onlyslightly curved in the direction between lateral edges 30, in order torestrict pivoting in that direction and thus stabilize the knee jointagainst sideward movement. The pivotal wear surface 16 is buff-polishedwith a diamond dust abrasive in order to reduce friction and wear.

As shown more clearly in FIG. 2, the face 18 which is opposite the wearsurface 16 is provided with a series of projections or lugs 20 andindentations or grooves 22 to assure a strong, stable mechanicalconnection of the prosthesis 10 with the natural bone of the femur ortibia. The pyrolytic carbon coating on the bone-joining face 18 havingthe lugs 20 and grooves 22 is not polished, but rather is permitted toretain the degree of roughness associated with its deposition. Inaddition, it may be oxidized to enhance attachment. This roughnessfosters the development of a strong mechanical joint between theprosthesis l and the natural bone through the growth of natural bonetissue thereto. The pyrolytic carbon deposited on the front and rearsurfaces 26 and 28 and on the side surfaces 24 of the prosthesis mayalso be polished in order to reduce the possibility of irritation oftissues which may come into sliding contact with these surfaces duringmovement of the joint.

The substrate 12 is preferably formed from a polycrystalline graphite,sold under the trade name POCO AXF graphite, which has a density ofabout 1.9 grams per cubic centimeter, and average crystallite size (L,of about 300A, and an isotropy on the Bacon scale of nearly 1.0. Thegraphite has a Youngs modulus of elasticity of 1.7 X 10 psi. Thesubstrate is formed in the shape of the prosthesis l0 and is coated witha layer of pyrolytic carbon about 500 microns thick which has a densityof about 1.9 gm/cm and a modulus of elasticity of 4 X 10' psi. Thecomposite prosthesis 10 has an effective modulus of elasticity which isin the range of that of natural living bone, which has a modulus betweenabout 2 X 10 and 4 X 10' psi.

For replacement of a damaged knee joint, the natural bone joint portioncorresponding to the prosthesis 10 is surgically removed, and theremaining end is shaped to provide a mating face that will fit ininterlocking relationship with the lug and groove containing face 18.The unpolished pyrolytic carbon coating 14 on the face 18 is conduciveto natural bone tissue growth and adherence thereto, and its presenceinduces acceptance of the replacement prosthesis as a functionallypermanent section of the natural bone. Because the prosthesis has arigidity closely approximating that of the natural bone, an excellentanchor is provided, and the prosthesis will remain permanently andperform like natural bone. Thus, since the modulus of elasticity of theprosthesis 10 is very close to that of natural bone, the bone and theprosthesis which is attached to it deform elastically in harmony so thatthe tendency for the device to work loose is greatly reduced.

It is contemplated that the prosthesis 10 would be employed with amating prosthesis; thus, for the design shown in FIGS. 1 and 2, theentire knee joint would be replaced by prostheses at the articulatingends of both the femur and the tibia. In use against a mating carbonpiece as described above, the polished arcuate surface 16 of thepyrolytic carbon coating has extremely good wear resistance so thatre-operation because of inflammation caused by dust resulting from wearis not necessitated. It is understood that if it is desired or necessaryto only replace one portion of the knee joint, (i.e., only that ofeither the femur or tibia), that the pivotal wear surface 16 may befabricated in a different shape to more beneficially function againstthe articulating end of the natural bone portion of the joint. In such acase, it is understood that the matching of physical properties ofnatural bone achieved through the use of the pyrolytic carbon-coatedgraphite substrate is superior to the mating of a natural bone end (orcartilage) into contact with, for example, a metal which hasa modulus ofelasticity which may be 10 times higher than that of the natural boneend. Where necessary to repair or replace a knee joint which requiresadditional replacement of other portions of natural bone, theillustrated prosthetic device may readily be increased in size asnecessary.

Shown in FIG. 3'is a prosthesis 40 for implant dentistry. The dentalprosthesis 40 is fabricated in the desired shape from a substrate 42 ofpolycrystalline POCO AXF graphite, such as described hereinabove, whichhas deposited thereon a pyrolytic carbon coating 44. The substrate 42 isfabricated so that it has a rectangular upper abutment 46 and a lowerbase section 54 having exterior spiral projections or threads 48 thereonso that the prosthesis may be screwed into a suitable hole drilled intothe jawbone in order to provide an immediate anchoringfor theprosthesis. The substrate is fabricated with an intermediate cylindricalportion 50 separating the upper abutment 46 and the base section 54 withthreads 48, so that when the prosthesis is screwed into a hole drilledin the jawbone until the end 52 of base section 54 contacts the bottomof the hole, the upper abutment 46 will. protrude the proper distancethrough the gum line.

The upper abutment 46 provides a stud-like surface to which a denturesuch as a porcelain tooth, can be readily mounted, for example, bystandard cements. Subsequent growth of natural bone tissue will morefirmly anchor the prosthesis into the jawboneThe porcelain tooth ispreferably affixed only after sufficient substrate 62 and a pyrolyticcarbon exterior coating 64. The prosthesis 60 has an upper abutment 66which is reinforced by a percent tungsten-90 percent tantalum alloy wire68 penetrating through the abutment 66 from a position slightly abovethe upper surface of the abutment, to a position extending into theintermediate portion 72 of the prosthesis. The wire 68 is inserted intoand suitably bonded to the substrate 62 prior to deposition of pyrolyticcarbon coating 64 thereon. After deposition of the pyrolytic carboncoating, longitudinal indentations or grooves 70 are machined into abase portion 74 of the prosthesis of sufficient depth to penetratethrough the pyrolytic carboncoating 64 and expose the substrate 62. Theprosthesis 60 has an intermediate portion 72 of sufficient length suchthat when the grooved base portion 74 is inserted into a suitablydrilled hole in the jawbone, the pyrolytic carbon-coated upper abutment66 and the wire 68 will protrude a sufficient distance so that aporcelain tooth may be adequately adhered thereto. The after-machinedgrooves 70 provide localized exposed regions of the porous substrateinto which bone tissue may grow, which may beused in combination withthe as-deposited surface roughness of the prosthesis to provideattachment to the jawbone through natural bone tissue growth.

The prosthesis has approximately the same modulus of elasticity as thenatural bone into which it is inserted so that it performs in harmonywith the natural bone without concentrating stress at the naturalboneprosthesis interface, which might cause the prosthesis to workloose. The wire 68, having pyrolytic carbon deposited thereon, providesstructural reinforcement in the upper portion of the prosthesis withoutaffecting the modulus of the base portion which is inserted into thenatural bone, and is particularly adapted for longer, narrowerartificial teeth which may be more difficult to anchor to theprosthesis, such as cuspids and incisors.

The pyrolytic carbon-coated dental prostheses have the outstandingstrength which is required for satisfactory performance under thedemanding conditions required of dental insert prostheses. While twopossible configurations of prostheses have been illustrated, it isapparent that others may be used as well. For example, the root may havea multitude of grooves, holes or crevices that were originally machinedintothe substrate (a special case of this is the helical spiral threadsillustrated in FIG. 3). A smooth-walled substrate might also'possibly beused, in which case one might rely completely on the as-depositedsurface roughness of the pyrolytic coating and the adherence of naturalbone tissue growth thereto to provide attachment. The porosity of thepyrolytic carbon may be varied depending upon the degree of natural boneadhesion'to the pyrolytic carbon which is desired in each case.

Alternately, the dental implant may be machined with the same profile asthe extracted tooth and is implanted in the cavity left after theextraction. In practice, a tooth may be extracted and an impression madeof the resulting cavity. The tooth may be replaced to retain the shapeof the cavity until the carbon prosthesis is fabricated. Then the toothis removed and the prosthesis implanted. The prosthesis is compatiblewith the natural tissue of the cavity.

Illustrated in FIG. 5 is a shock-absorbing hip joint prosthesis whichhas a ball section 82 fabricated from a polycrystalline carbon substrate84 having a coating 86 of pyrolytic carbon thereon and which is shapedso as to be an effective replacement for the natural bone portion of thejoinLThe exterior portion of the pyrolytic carbon coating 88 is polishedto a high gloss by buffing with a diamond dust abrasive to provide anexceptional wear surface and to reduce friction. The ball 82 is fastenedto a metal shank or strut 90 by means of a suitable elastomer interlayer92 which is adhesively bonded to each.

One common problem with the metal-hip joint prostheses has been theirlack of the shock-absorbing characteristics of natural bone; the use ofthe ball 82 which is a carbon substrate coated with pyrolytic carbonreduces this deficiency, but additional cushioning of the joint may bedesirable. In this regard, the elastomer interlayer 92 not only providesbonding between the ball and the metal strut 90 but also serves ashock-absorbing function. The shock-absorbing layer 92 may be fabricatedfrom polyethylene or other suitable rubbery material, and it may beprotected from body juices by a suitable seal 98 which is held in slightcompression against both the ball and the metal strut by biasingtensionproduced by the elastomer fastening.

At its end opposite the ball 82, the strut 90 angles to form a rod 100which is inserted into and bonded to a tapered sleeve 102 fabricatedfrom polycrystalline carbon, which may have an outer coating ofpyrolytic carbon 104. A fastener 106 may be used on the end of the rod100 to secure the sleeve thereto. The tapered sleeve 102 with thepyrolytic carbon coating 104 thereon is eminently suited for insertioninto the femur to provide an excellent bond by means of adhesion withnatural bone tissue.

Alternatively, the entire prosthesis 80 may be fabricated from a carbonsubstrate having deposited thereon a coating of pyrolytic carbon, eitherwith or without the shock'absorbing feature of the illustratedembodiment.

In certain instances where a segment of bone is missing due to accidentor surgery, there is at present no satisfactory method other than bonehomograft which is available to join the remaining portions of the boneor replace the missing bone segment. The structure of natural bone isfibrous with channels running longitudinally therethrough. Illustratedin FIG. 6 is a tubular bone segment replacement framework 120. Theframework is formed by rolling a strip of woven metallic screen (such as10 percent tungsten-90 percent tantalum alloy screen) into tubular form,followed by deposition thereon of a pyrolytic carbon coating v124. Thus,bone cells which are formed within the framework 120 have sufficientaccess to'nutrients to become calcified (i.e., strong and supportive)via the longitudinal channels provided between and through the coils ofthe roller screen 120.

In the illustrated embodiment, the center of the framework is hollow,and the tightness of the roll, the mesh size of the screen 122 and thethickness of the pyrolytic coating 124 deposited thereon are variablesthat are determined by the particular application. The deposition of thepyrolytic carbon coating I24 upon the screen 122 is accomplished in sucha manner that the physical characteristics of the screen are largelypreserved, i.e., the regular geometry and porosity of the screenremains.

In use, the framework 120 abuts the bone segments to be joined (or thebone segment) and is fastened thereto by suitable means. For example,the framework 120 may be provided with a central elongated core portion126 for insertion into the marrow portion-of natural bone, asillustrated in FIG. 6. FIG. 7 illustrates the framework prosthesis 120in position between two segments of natural bone 130.

Since the pyrolytic carbon coating 124 is conducive to natural bonegrowth, and since the regular geometry of the framework 120 is'such thatbone cells may grow throughout the framework while being provided withsufficient nutrients, the framework 120 provides a means of permittingnatural bone tissue growth to replace or repair missing or diseased bonesegments, partially through natural processes. Other more complicatedshapes may be formed by laying up formed sheets of screening so thatmany layers are used, and fastening these together for coating. Filamentwinding procedures which employ the winding of single or multiplestrands, or other suitable processes for forming structural networks offibers may be used. The resulting shapes, when coated with pyrolyticcarbon, should have sufficient porosity and oriented channels so thatproper healthy bone can form within and around it. Suitable surgicalprocedures, such as seeding the framework with small portions of naturalbone segments from the patient may be developed through the use of suchdevices to greatly speed up the natural replacement process.

Screening or fiber segments having pyrolytic carbon coatings depositedthereon may also be employed with other prosthetic devices such as thoseformed from a carbon substrate to aid in the formation of a strong bondbetween the prosthetic device and natural bone.

Pyrolytic carbon coated on a metal wire substrate may also have otherprosthetic applications. For example, a prosthesis for insert dentistrymay be provided which comprises a bridge constructed of a suitablerefractory wire having an abutment thereon. The whole device is coatedwith pyrolytic carbon and placement is made, not into the jawbone, butrather so that the prosthesis straddles the jawbone. It is implantedunder the skin so that only the small abutment protrudes through theskin; in this case also, the carbon coated framework grows to the boneto provide a strong support for the abutment. Subsequently, a denturesuch as a porcelain tooth or other dental prosthesis is fastened to theabutment, which provides firm support. Such implants are useful forsupporting extracorporeal dental bridgework so that additionalmechanical strain need not be placed on remaining teeth by fastening thebridgework to them.

Although the foregoing examples disclose the best modes presentlycontemplated by the inventors for carrying out their invention, itshould be understood that these examples are only illustrative and donot constitute limitations upon the invention which is defined by theclaims appearing at the end of the specification. The following Examplesfurther illustrate the fabrication and use of specific embodiments ofthis invention.

14 EXAMPLE I A knee-joint prosthesis such as illustrated in FIGS. 1

and 2 is machined from polycrystalline graphite (POCO AXF graphite). Themachined substrate is designed to be of sufficient size for repair ofthe knee joint of an average, adult human male. The substrate is placedin a rotating wire cage within a reaction chamber that contains inaddition a bed of particles. The substrate fits loosely in the wire cageso that, when it is rotated, the point or points of contact of thesubstrate with the wire cage will change as they rotate, so that thepyrolytic carbon coating will be evenly deposited.

The reaction chamber is heated to a temperature of about I350C. When thetemperature of the substrate reaches about l350C., a flow of helium gasand propane, such that there is a partial pressure of propane of 0.4(total pressure one atmosphere), is introduced into the reaction vesselso that it is directed against the surfaces of the substrate, which isbeing rotated in the wire cage. The propane decomposes under theseconditions to deposit a dense isotropic pyrolytic carbon coating uponall of the surfaces of the polycrystalline graphite substrate. Underthese coating conditions, the carbon deposition rate is about 5 micronsper minute, and the propane gas flow is continued until an isotropicpyrolytic carbon coating about 500 microns thick is deposited on thesubstrate. After the desired thickness of pyrolytic carbon is depositedon the outside of the prosthesis substrate, the propane gas flow isterminated. The coated substrate is cooled fairly slowly in the heliumgas stream, and it is then removed from the reaction chamber.

The wear surface and the front, rear, and side surfaces of theprosthesis are buffed to a high gloss by using a diamond dust abrasive.The as-deposited surface roughness of the pyrolytic coating is allowedto remain on the bone-joining face containing the lugs and grooves, andthis facilitates the establishment of a strong bond between theprosthesis and the natural bone. Measurement shows that the pyrolyticcarbon has a density of about 1.9 gm/cm, an L. of about 30 A. and a BAFof about 1.1.

EXAMPLE ll Dental prosthesis substrates like those illustrated in FIG. 3are machined from artificial polycrystalline graphite (POCO graphite).The prostheses have squared abutments about 4 mm. high and 2 mm. wide,an intermediate portion about 6 mm. long and 3 mm. in diameter, and athreaded base portion about 8 mm. long and 4 mm. in diameter at theoutside of the threads.

The artificial graphite employed as a substrate has a coefficient ofthermal expansion of about 8 X l0 /C. when measured at 50C. The dentalprosthesis substrates are coated with pyrolytic carbon using a fluidizedbed coating apparatus which includes a reaction tube having a diameterof about 3.8 centimeters that is heated to a temperature of about 1350C.A flow of helium gas sufficient to levitate a number of the relativelysmall prostheses along with the bed is maintained upwardly through theapparatus.

A number of prostheses are coated together with a charge of about 50grams of zirconium dioxide particles which have diameters in the rangeof about 150 to 250 microns. The particles are added along with theprostheses to provide a deposition surface area of the desired amount,relative to the size of the region of the reaction tube wherein thepyrolysis occurs, inasmuch as the relative amount .of available surfacearea is another factor which influences the physical characteristics ofthe resultant pyrolytic carbon. When the temperature of the particleswhich are levitated within the reaction tube reaches about l350C.,propane is admixed with the helium to provide an upwardly flowing gasstream having a total flow rate of about l. per minute and having apartial pressure of propane of about 0.4 (total pressure 1 atmosphere).The propane decomposes under these conditions and deposits a denseisotropic pyrolytic carbon coating upon all of the articles in thefluidized bed. Under these coating conditions, the carbon depositionrate is about 5 microns per 'minute. The propane gas flow is continueduntil an isotropic pyrolytic carbon coating about 300 microns thick isdeposited on the outside of the prostheses substrates. At this time, thepropane gas flow is terminated and the coated articles are cooled fairlyslowly in the helium gas and then removed from the reaction tube coatingapparatus.

Photomicrog-raphs taken of the pyrolytic carbon coating of theprostheses using an electron-scanning microscope show the as -depositedsurfaces to have a coral-like appearance. The dental prosthesis may bescrewed into suitably drilled holes in a living jawbone to providean'abutment to which an artificial tooth or a denture may be affixed.The pyrolytic carbon coating is conducive to bone growth, and thenatural formation of bony tissue around and attached to the prosthesisin time will provide a very firm anchor for the prosthesis. In addition,the prosthesis matches the modulus .of elasticity of natural bone, andthere is little or no tendency for the prosthesis to work loose undersevere loads after the prosthesis has been heated in place.

EXAMPLE III A sheet of metal screen is selected which is composed of analloy of IO percent tungsten-90 percent tantalum, which is woven fromwire having a diameter of 0.005 and a mesh of such wires per inch. Asingle, T-shaped piece is cut from the screen having a cross piecedimension of 2% inches by 6 inches, and a base piece dimension of 1%inches wide, which is equal to the length of the missing bone segment.The wire screen is rolled up from the cross piece toward the base sothat a tube such as that illustrated in H68. 6 and 7 is formed, whichhas an outside diameter equivalent to that of the bone to be replaced.The rolled tube has an outer layer of screen about 1% inches long, and acentrally located internal tubular portion of the rolled screen which is2% inches long. The rolled screen is fastened in this position andstructurally interlaced by means of fine wires of the sametungsten-tantalum alloy, and any sharp ends of wire from the screen orthe fastening wires are bent inwardly and crimped. The rolled tube isplaced loosely on a rotating mandrel in a reaction tube having adiameter of 4 inches through which a flow of helium is maintained andwhich is heated to a temperature of about 1300C.

in addition to the wire tube, a charge of about grams of zirconiumdioxide particles which have diameters in the range of about to 250microns is introduced to modify the relative amount of available surfacearea for deposition. When the screen reaches the temperature of thereaction tube, propane is added to the upward flow of helium past thescreen on the rotating mandrel. The gas stream has a partial pressure ofpropane of 0.4 (total pressure one atmosphere). The propane decomposesunder these conditions to deposit a dense isotropic pyrolytic carboncoating upon the screen. The propane gas flow is continued until anisotropic pyrolytic carbon coating about 200 microns thick is depositedon the outside of the wire screen substrate. This degree of depositionleaves intact the mesh nature of the screen, while strengthening it, andproviding the physiologically beneficial pyrolytic carbon coatingthereon.

The prosthesis may be used to replace a l k inch section of damaged ordiseased bone in a manner such as illustrated in FIG. 7. Thus, A inchlengths of the bone segment to be joined are bored out so that the innertube portions may he slipped into the bone ends. The frameworkprosthesis may be affixed, for example by small pins or small pyrolyticcarbon-coated screws.

EXAMPLE IV A knee joint prosthesis is prepared exactly as Example 1,except that different structure is provided for achieving a firm bondbetween the prosthesis and the natural bone to which it is attached.Instead of a series of lugs and grooves as in the prosthesis illustratedin FIGS. 1 and 2, the substrate'is provided with only one lug locatedcentrally in the bone joining face; however, a strip of metal screen (90percent-tungsten-IO percent tantalum alloy is wrapped around and bondedto the periphery of the substrate so that one side of the screen doesnot extend quite up to the wear surface, but on the other side overlapsbeyond the bone joining face a distance of about inch. Deposition of thepyrolytic carbon coating as in Example I serves to further bond thescreen to the substrate and to weld the entire prosthesis togetheras aunit. Following deposition of the pyrolytic carbon coating as in ExampleI, the wear surface is highly polished by buffing with a diamond dustabrasive. I

The prosthesis may be implanted by machining the bone face it is to abutso that it receives the lug, and by slipping the pyrolytic carbon-coatedscreen over the outside of the bone end. The screen may be fastened bymeans of screws or pins if desired; subsequent growth of bone tissue tothe prosthesis, including the bone joining face and the overlappingscreen, provides a stable bond. The technique illustrated in thisExample may also be employed in the joining of other types of prosthesesto natural bone segments.

Although the figures and examples illustrate only certain specificembodiments of this invention, such as the specific types of prostheticdevices, specific pyrolytic carbon coating methods, fastening methodsand substrate materials, it is contemplatedthat other embodiments may beemployed also. Thus, in addition to knee and hip joints, other joints,such as finger, elbow and shoulder joints, may be repaired or replacedby pyrolytic carbon coated prostheses. ln addition, long bones, otherssuch as phlanges, vertebrae, etc., may be repaired or replaced in wholeor in part.

Although the examples particularly describe the deposition of pyrolyticcarbon through the use of propane, it is understood that otherhydrocarbons, or mixtures of hydrocarbons, may be employed to depositthe pyrolytic carbon coating on the substrate. The variables 'of thedeposition process itself may be employed to vary the properties of thepyrolytic carbon coating, or to accommodate the size, temperature-stablerange,

, or structure of the substrate material.

A number of specific methods of fastening the prosthetic device tonatural bone tissue have been demonstrated. The pyrolytic carbon coatingis believed to have a particular degree of as-deposited surfaceroughness or surface porosity which aids in the adherence of growingbone tissue thereto. In support of this theory by which, however, it isnot intended that the invention be limited, photomicrographs madethrough the use of an electron scanning microscope reveal that preferredpyrolytic carbon deposits have a surface which looks similar, under highmagnification, to coral. The porosity is only superficial; the bulk ofthe coating is dense with a preferred range of density from about 1.9g./cm to about 2.2 g./cm

Although a wide range of substrate materials may be employed, thosewhich have a modulus of elasticity near that of natural bone areparticularly preferred for use in prostheses for bulk replacement ofbone. Artificial graphites are particularly preferred. For frameworkprostheses, various refractory fibers and wires which have been formedin woven, wound and nonwoven structure may be employed as substrates.Prostheses made from such substrates are also useful for providingsupport for mending shattered or diseased bones, which support does notcorrode, weaken or inflame, as can prior art devices, and which mayremain permanently as a structural part of the healed bone segment.Where it is desired that high modulus metallic fibers or screens not beused, lower modulus fibers, such as carbon fibers, cloth or screen, maybe coated with pyrolytic carbon to provide a lower modulus frameworkwhich has high strength and the biological compatibility and otherproperties of pyrolytic carbon.

Various features of the invention are set forth in the following claims.

What is claimed is:

-l. A composite orthopedic prosthetic device for repair or replacementof bone structure in a living body, comprising a refractory substrate ofsuitable predetermined shape and a vapor-deposited pyrolytic carboncoating on said substrate, said pyrolytic carbon coating beingcompatible with living tissue and having a bulk density of at leastabout 1.5 grams per cubic centimeter, a crystallite size no greater thanabout 200 A, a thickness of at least about 25 microns, and a BaconAnisotrophy Factor of not more than about 2.0, and said pyrolytic carboncoating having an as-deposited surface porosity on at least a portion ofits surface inyide sufficient access to nutrients for the rowth of living bone tissue therethrough, and wherr ain said substrate is afiberaggregate form. v

3. A prosthetic device according to claim 2 wherein said substrate ismade of refractory metal selected from the group consisting of tungsten,tantalum, molybdenum, and alloys thereof.

4. A prosthetic device according to claim 2 wherein said substrate ismade of carbon fiber.

5. A prosthetic device according to claim 1 wherein said substrate has athermal coefficient of expansion of from about 3 X l0'/C. to about 8 Xl0'/C.

6. A prosthetic device according to claim 5 wherein the substrate has amodulus of elasticity of between about 1 X 10' psi. to about 5 X 10 psi.

7. A prosthetic device according to claim 6 wherein the substrate isartificial graphite having a modulus of elasticity between about 1.7 X10' psi. and about 4 X 10 psi. such that the pyrolytic carbon coatedsubstrate has a modulus of elasticity approximating that of naturalbone.

8. A prosthetic device according to claim 7 wherein said substrate isisotropic polycrystalline graphite having an average crystallite size ofabout 300 A, a density of about 1.9 grams per cubic centimeter, and amodulus of elasticity of about 1.7 X 10 psi.

9. A prosthesis according to claim 7 wherein said density of saidpyrolytic carbon coating is between about 1.9 and about 2.2 grams percmi.

10. A prosthetic device according to claim 7 wherein said device is ajoint prosthesis having a bone-joining surface and an articulating wearsurface, and wherein the pyrolytic carbon coating on the wear surface ispolished to reduce friction and improve wear characteristics 11. Aprosthetic device according to claim 10 wherein the bone-joining surfacehas at least one projection adapted to fit in interlocking relationshipwith a mating, natural bone surface.

12. A prosthetic device according to claim 7 wherein the prostheticdevice is a dental prosthesis having a base portion adapted to beinserted into a jawbone, an abutment for attaching a tooth or otherdenture, and an intermediate portion between the abutment and baseportion of sufficient length such that upon insertion the abutmentprotrudes the proper distance from the jawbone for attachment of thetooth or other denture.

13. A dental prosthesis according to claim 12 wherein the base portionof the prosthesis is formed with indentations penetrating through thepyrolytic carbon coating into the substrate.

14. A prosthetic device according to claim 1 wherein the surface of saidpyrolytic carbon coating is provided with oxygen-bearing polar groups toenhance the attachment of bone or other tissue thereto.

* i i i

2. A prosthetic device according to claim 1 wherein the prosthesis is aframework prosthesis adapted to provide sufficient access to nutrientsfor the growth of living bone tissue therethrough, and wherein saidsubstrate is a fiber aggregate form.
 3. A prosthetic device according toclaim 2 wherein said substrate is made of refractory metal selected fromthe group consisting of tungsten, tantalum, molybdenum, and alloysthereof.
 4. A prosthetic device according to claim 2 wherein saidsubstrate is made of carbon fiber.
 5. A prosthetic device according toclaim 1 wherein said substrate has a thermal coefficient of expansion offrom about 3 X 10 6/*C. to about 8 X 10 6/*C.
 6. A prosthetic deviceaccording to claim 5 wherein the substrate has a modulus of elasticityof between about 1 X 106 psi. to about 5 X 106 psi.
 7. A prostheticdevice according to claim 6 wherein the substrate is artificial graphitehaving a modulus of elasticity between about 1.7 X 106 psi. and about 4X 106 psi. such that the pyrolytic carbon coated substrate has a modulusof elasticity approximating that of natural bone.
 8. A prosthetic deviceaccording to claim 7 wherein said substrate is isotropic polycrystallinegraphite having an average crystallite size of about 300 A, a density ofabout 1.9 grams per cubic centimeter, and a modulus of elasticity ofabout 1.7 X 106 psi.
 9. A prosthesis according to claim 7 wherein saiddensity of said pyrolytic carbon coating is between about 1.9 and about2.2 grams per cm3.
 10. A prosthetic device according to claim 7 whereinsaid device is a joint prosthesis having a bone-joining surface and anarticulating wear surface, and wherein the pyrolytic carbon coating onthe wear surface is polished to reduce friction and improve wearcharacteristics
 11. A prosthetic device according to claim 10 whereinthe bone-joining surface has at least one projection adapted to fit ininterlocking relationship with a mating, natural bone surface.
 12. Aprosthetic device according to claim 7 wherein the prosthetic device isa dental prosthesis having a base portion adapted to be inserted into ajawbone, an abutment for attaching a tooth or other denture, and anintermediate portion between the abutment and base portion of sufficientlength such that upon insertion the abutment protrudes the properdistance from the jawbone for attachment of the tooth or other denture.13. A dental prosthesis according to claim 12 wherein the base portionof the prosthesis is formed with indentations penetrating through thepyrolytic carbon coating into the substrate.
 14. A prosthetic deviceaccording to claim 1 wherein the surface of said pyrolytic carboncoating is provided with oxygen-bearing polar groups to enhance theattachment of bone or other tissue thereto.